1. Field of the Invention
The present invention generally relates to a reading apparatus employed in facsimile systems. More specifically, the invention relates to past record correcting/reading apparatus in which an image sensor is employed to produce photoelectric conversion outputs and the photoelectric conversion outputs are converted into signals in accordance with transmission systems.
2. Description of the Related Art
In general, the reading apparatus as described in the preamble is arranged by an image sensor, an amplifier, a comparator, a lens, a light source and so on. In the reading apparatus, an original document is illuminated by light coming from the light source, and then the light reflected from the original document is focused via the lens onto the image sensor. A single array, or a plurality of arrays of so-called "photosensitive elements" are arranged in a line form along a main scanning direction. The photosensitive elements have such characteristics that electromotive power, resistance, and charged electrons vary with the amount of light incident thereon. The main scanning of the image sensor is accomplished by sequentially, electronically switching the photoelectric conversion signal of the respective photosensitive elements and successively deriving them therefrom. The sub-scanning of the image sensor is performed by relatively transporting the original document, lens, or image sensor by way of mechanical means or optical means.
The photoelectric conversion signals from the image sensor are conducted to the amplifier to be amplified. The amplified signals are derived from the amplifier as photoelectric conversion outputs Vs.
The processing mode for these photoelectric conversion outputs Vs varies in accordance with the transmission systems of the facsimile signals, i.e., analog transmission systems, or digital transmission systems. In case of analog transmission system, the photoelectric conversion outputs Vs may be directly conducted to a modulator for transmission purposes, whereas in the event of digital transmission system, they may be quantized via an analog-to-digital (A/D) converter and if necessary, further be coded for transmission purposes. That is to say, according to the A/D conversion of the digital transmission system, the photoelectric conversion outputs Vs are binary-quantized, based upon a reference voltage V.sub.R (slice level) of a comparator, into binary-coded signals that are derived from the output terminals as output signals of the A/D converter, assuming that photoelectric conversion outputs are indicated by Vs.
A description will now be made of the A/D conversion in more detail.
When slow photo-response image sensors such as CdS (cadmium sulfide) image sensors are employed as the image sensor, rising and falling portions of the photoelectric conversion outputs Vs represent gentle curves. Accordingly, assuming that the scanning time period is equal to .tau., either the rise time, or fall time must be shorter than, or equal to the scanning time period. Then, since the rise and fall times are substantially equal to .tau., the binary quantization is performed by fixing the slice level to approximately 50% (percents) of the photoelectric converted output voltage.
In the conventional reading apparatus, the scanning time period .tau. is selected to be longer than the rise time, or the fall time of the photoelectric conversion output, and the binary-quantized signal is obtained by the above-described means with maintaining substantially completely the original image information.
The conventional reading apparatus has, however, disadvantage that resolution in the sub-scanning direction is deteriorated, an image quality is deteriorated, and a limitation is provided with the scanning speed due to the previous reasons, even if such conventional means is utilized. This is because a longer time is required when the output of the image sensor reaches the photoelectric conversion output level determined by the reflections of the original document, due to the slow photo-response characteristic of the image sensor. The deteriorations of both the resolution and image quality are emphasized particularly in the high speed scanning. For instance, if the scanning rate is twice as the above scanning rate, i.e., the scanning time period being .tau./.sub.2, the rising and falling characteristics of the photoelectric conversion output with respect to the image information are considerably deteriorated, as compared with the scanning time period of .tau.. As a result, the photoelectric conversion outputs concerning small image signals do not exceed the slice level, so that the image information for the small image signals will be lost. The waveform of the resultant binary-coded output signal is very different from that of the original image information, resulting in serious deteriorations of the image resolution and quality.
It is apparent that the higher the scanning rate increases, the more the small signal portions are lost. In other words, there is a limitation in the scanning rate. Consequently, it should be understood that no binary-quantized signal output analogous to the original image information is obtained unless the image scanning is implemented at a constant scanning time period longer than the rise and fall times of the employed image sensor.